1. Field of the Invention
The present invention relates to a switching device and, more specifically, to a switching device suitably used for normal/reverse rotation switching, rotational speed switching, and the like typically in power tools such as an electric drill and an electric screwdriver.
2. Description of the Related Art
FIG. 29 shows a circuit configuration of the main part of a conventional trigger switch (switching device) which is used in a power tool such as an electric drill. FIG. 30 is its vertical sectional view, and FIG. 31 is its partially cutaway plan view.
The trigger switch is provided with the following components. A pair of changeover switches 73 and 74 operate in link motion to switch the connections of both terminals of a DC motor 51 for drill blade driving in response to a manipulation on a switching lever 50 for normal/reverse rotation switching of the DC motor 51. A brake switch 54 brakes the DC motor 51 by short-circuiting both terminals thereof when a manipulation lever (trigger) 53, which is pulled by fingers for drill blade rotary driving, is at the free position, i.e., non-manipulation position. A first switch 56 connects a DC power supply to the DC motor 51 via a FET 55 for rotational speed control. A second switch 57 short-circuits the DC motor 51 with the DC power supply to rotate the DC motor 51 at the maximum speed when the manipulation lever 53 is pulled to the full stroke. A diode 58 is also provided.
As shown in FIG. 30, the brake switch 54 is composed of a braking movable contact 61 mounted on a manipulation shaft 60 which is urged in the direction of arrow C by means of a return spring 59, a coil spring 62 for urging the movable contact 61 in the direction of arrow C, and top and bottom braking fixed contacts 63 and 64 which are mounted on a case. When the manipulation lever 53 is at the free position where it is not pulled by fingers in the direction of arrow D in FIG. 30, the braking movable contact 61 is in pressure contact with the braking fixed contacts 63 and 64, whereby the brake switch is on to brake the DC motor 51.
The first switch 56 is composed of a fixed contact 65 which is mounted on a top portion of the case and a movable piece 68 which is urged by a coil spring 66 so that a movable contact 67 is brought into pressure contact with the fixed contact 65. At the free position, the free end of the movable piece 68 is placed on a protrusion 60b at a top portion of a plunger 60a of the manipulation shaft 60, whereby the contacts 65 and 67 are separated from each other and hence the first switch 56 is in an off-state.
The second switch 57 is composed of a fixed switch 69 which is mounted on a bottom portion of the case and a movable piece 72 which is urged by a coil spring 70 so that a movable contact 71 is brought into pressure contact with the fixed contact 69. At the free position, the free end.of the movable piece 72 is placed on a protrusion 60c at a bottom portion of the plunger 60a of the manipulation shaft 60, whereby the contacts 69 and 71 are separated from each other and hence the second switch 57 is in an off-state.
The first and second changeover switches 75 and 76, which are linked with each other to operate to switch the connections of both terminals of the DC motor 51 in response to a switching manipulation on the switching lever 50, are composed of fixed contacts 75 and 76 connected to the respective terminals of the DC motor 51, changeover contacts 77 and 78 to effect a changeover operation in response to a manipulation on the switching lever 50, fixed contacts 79 and 80 connected to the positive side of the DC power supply, and fixed contacts 81 and 82 to be connected to the negative side of the DC power supply via the first switch 56 and the FET 55 or the second switch 57.
The switching lever 50 (manipulating section) is pivotable about a pivot 83 in accordance with a switching manipulation. As shown in FIGS. 30 and 31, a protrusion 84a of a changeover cam 84 (changeover section) which is provided with the changeover contacts 77 and 78 of the first and second changeover switches 73 and 74 is engaged with an end portion of the switching lever 50. In accordance with a switching manipulation on the switching lever 50 which acts on the changeover cam 84 via the protrusion 84a, the changeover cam 84 pivots about a pivot 85 which is different from the pivot 83 of the switching lever 50. As shown in FIG. 31, the fixed contacts 75, 76, and 79-82 of the first and second changeover switches 73 and 74 are disposed around the changeover cam 84. In FIG. 31, reference numerals 90 and 91 are a radiation plate and a screw, respectively.
FIGS. 32A-32C show connection states between the changeover contacts 77 and 78 of the changeover cam 84 and the fixed contacts 75, 76, and 79-82; FIG. 32A shows a neutral state, FIG. 32B shows a normal rotation state, and FIG. 32C shows a reverse rotation state.
When the switching lever 50 in in the neutral state, the changeover contacts 75 and 76 of the changeover cam 84 are respectively connected to only the fixed contacts 75 and 76 which are connected to the respective terminals of the DC motor 51. When switching in made from the neutral state to the normal rotation state by a switching manipulation on the switching lever 50, the changeover cam 84 rotates to connect the fixed contacts 75 and 79 (80) via the changeover contact 77 while connecting the fixed contacts 76 and 81 (82) via the changeover contact 78, to establish the intended normal rotation state. On the other hand, when the normal rotation state is selected by manipulating the switching lever 50 in the opposite direction, the changeover cam 84 rotates to connect the fixed contacts 75 and 81 (82) via the changeover contact 77 while connecting the fixed contacts 76 and 79 (80) via the changeover contact 78, to establish the intended reverse rotation state.
Next, the operation of the above conventional trigger switch will be described.
It is now assumed that, for instance, the changeover switches 73 and 74 are in the state of FIG. 32B, that is, the normal rotation state is selected by manipulating the switching lever 50.
First, at the free position where the manipulation lever 53 is not pulled by fingers at all, the brake switch 54 is on while the first and second switches 56 and 57 are off, as described above.
When the manipulation lever 53 is pulled from the free position, after a play stroke the braking movable contact 61 of the manipulation shaft 60 is separated from the braking fixed contacts 63 and 64 to turn off the brake switch 54. Then, the free end of the movable piece 68 of the first switch 56 goes over the protrusion 60b at the top portion of the plunger 60a, so that the movable contact 67 rotates to contact with the fixed contact 65 (see FIG. 33), to thereby turn on the first switch 56. Supplied with power in this manner, the DC motor 51 starts to rotate in the normal direction. Further, in accordance with the pulling stroke of the manipulation lever 53, a brush 88 which is provided in the plunger 60a of the manipulation shaft 60 slides on a resistor of a circuit board (not shown), whereby a current corresponding to a slide position is supplied to the DC motor 51 via the FET 55 for rotational speed control. Thus, the DC motor 51 rotates at a rotational speed corresponding to the pulling stroke of the manipulation lever 53.
When the pulling stroke of the manipulation lever 53 reaches a predetermined value, the free end of the movable piece 72 goes over the protrusion 60c at the bottom portion of the plunger 60a of the manipulation shaft 60, so that the movable contact 671 rotates to contact with the fixed contact 69, to thereby turn on the second switch 57. Since the DC motor 51 is short-circuited with the DC power supply, the DC motor 51 rotates at the maximum speed.
On the other hand, when pulling of the manipulation lever 53 is released, the return spring 59 causes the manipulation shaft 60 to move in the direction of arrow C to effect an operation opposite to that when the manipulation lever 53 is pulled. That is, after the second switch is turned off, the first switch 56 is turned off to cut off power from the power supply and then the brake switch 54 is turned on to short-circuit both terminals of the DC motor 51 to thereby brake it.
When the reverse rotation state is selected by the normal/reverse rotation switching lever 50, the DC motor 51 rotates in the reverse direction in a manner similar to the above.
In the conventional trigger switch described above, as shown in FIG. 30, the first and second switches 56 and 57 (main switch section) includes the two movable pieces 68 and 72, the two coil springs 66 and 70 for urging the movable pieces 68 and 72, a terminal board 89 for connecting and supporting the movable pieces 68 and 72. Having so large a number of parts, the first and second switches 56 and 57 are not easy to assemble and costly.
As described above, the mechanism for normal/reverse rotation switching of the DC motor 51 as a load is constituted of individual parts of the switching lever 50 (manipulating section) and the changeover cam 84 (changeover section) which rotates in response to a switching manipulation on the switching lever 50. Therefore, this mechanism requires a number of assembling steps and hence is costly.
Further, as shown in FIG. 31, the radiation plate 90 for radiating heat from the FET 55 (heat generating element) is closely fixed to the FET 55 (located inside the case) by fastening with the screw 91 though an opening of the case. This mechanism is not easy to assemble either and requires a screw for fastening.